1. Field of the Invention
The present invention relates to a digital optical communication device and a method for transmitting and receiving data by infrared ray, and particularly to a digital optical communication device and a method for communication with the intensity of emitted light controlled in transmitting data.
2. Description of the Background Art
The conventional optical communication system is roughly classified into a communication system using a subcarrier wave and a communication system without using the subcarrier wave. The subcarrier wave refers to a carrier wave which is generated in a pseudo way by turning on/off the light at a certain cycle. A rectangular wave or a sine wave of the light which is simply turned on/off is often substituted for the subcarrier wave. A method of transmitting data for communication without using the subcarrier wave by changing the waveform according to a certain rule depending on the data is called a baseband communication system. On the other hand, a method of transmitting data for communication by changing any of the amplitude, phase and frequency of the subcarrier wave depending on the data is called a carrier band communication system. Among carrier band modulation systems, the simplest one changes the amplitude. Such a system is referred to as an ASK (Amplitude-Shift Keying) system.
FIG. 1 shows a format of a packet employed in a conventional optical communication system. An optical communication device on the transmitter side generates a packet by adding a start flag (STA) to the head of a data field and adding a stop flag (STO) to the end of the data field and transmits the packet. An optical communication device on the receiver side detects the start flag and performs a reception process by recognizing as data the field extending from the start flag to the stop flag. As shown in FIG. 1, an auto gain control (AGC) field and a preamble (PRE) field may be added in front of the start flag. Further, a CRC (Cyclic Redundancy Check) may be added in order to detect any error in a transmission channel.
FIG. 2A is a schematic block diagram showing a structure of a transmitter section of a conventional baseband optical communication device. The transmitter section includes a coding circuit 901 coding a transmission data 903 and outputting the coded signal, and an optical transmission circuit 902 converting a baseband transmission electric signal 904 supplied from coding circuit 901 to an optical signal (transmission optical signal 905) and outputting the converted signal.
FIG. 2B is a schematic block diagram illustrating a structure of a receiver section of the conventional baseband optical communication device. The receiver section includes an optical reception circuit 910 which receives an optical signal (reception optical signal 913) from any external source, converts the signal to an electric signal and outputs it, a clock regeneration circuit 911 which extracts a clock component from the reception electric signal 914 supplied from optical reception circuit 910 and outputs the clock component, and a decoding circuit 912 which regenerates a reception data 916 from the regenerated clock 915 supplied from clock regeneration circuit 911 and reception electric signal 914 supplied from optical reception circuit 910. Optical reception circuit 910 converts reception optical signal 913 to the electric signal, performs processes such as noise elimination, amplification, waveform reshaping, and thereafter outputs the signal as reception electric signal 914.
FIG. 3A is a schematic block diagram illustrating a structure of a transmitter section of a conventional ASK optical communication device. The transmitter section includes a coding circuit 901 coding a transmission data 903 and outputting the coded signal, a modulation circuit 920 which performs ASK modulation by superimposing a subcarrier wave on a baseband transmission electric signal 904 supplied from coding circuit 901, and an optical transmission circuit 902 which converts an ASK transmission electric signal 921 supplied from modulation circuit 920 to an optical signal (transmission optical signal 905) and outputs the optical signal.
FIG. 3B is a schematic block diagram illustrating a receiver section of the conventional ASK optical communication device. The receiver section includes an optical reception circuit 930 which converts an optical signal (reception optical signal 913) received from any external source to an electric signal and outputs the electric signal, a clock regeneration circuit 911 which extracts a clock component from the reception electric signal 914 supplied from optical reception circuit 930 and outputs the clock component, and a decoding circuit 912 which regenerates a reception data 916 from the regenerated clock 915 supplied from clock regeneration circuit 911 and reception electric signal 914 supplied from optical reception circuit 930. Optical reception circuit 930 converts reception optical signal 913 to the electric signal, performs processes such as noise elimination, amplification, waveform shaping (elimination of subcarrier wave), and thereafter outputs the electric signal as reception electric signal 914.
FIG. 4 illustrates a circuit structure of optical transmission circuit 902 of the conventional ASK optical transmission device shown in FIG. 3A. Optical transmission circuit 902 includes a light emitting device 935, and a drive circuit 936 which drives light emitting device 935. Drive circuit 936 includes a transistor Q1 having its emitter terminal connected to the ground, a resistor element R1 connected to the base terminal of transistor Q1, and a resistor element R2 connected between light emitting device 935 and the collector terminal of transistor Q1. Light emitting device 935 is formed of a light emitting diode (LED), a laser diode (LED) or the like.
ASK transmission electric signal 921 supplied from modulation circuit 920 is supplied to the base terminal of transistor Q1 via resistor element R1. When ASK transmission electric signal 921 is at a high level (hereinafter referred to as H level), transistor Q1 is turned on to allow electric current to flow through light emitting device 935 and accordingly light is emitted. When ASK transmission electric signal 921 is at a low level (hereinafter referred to as L level), transistor Q1 is turned off and accordingly, no electric current flows through light emitting device 935 and no light is emitted. Optical transmission circuit 902 thus converts ASK transmission electric signal 921 to transmission optical signal 905.
FIG. 5 is a schematic diagram illustrating a structure of optical reception circuit 930 of the conventional ASK optical reception device. Optical reception circuit 930 includes a light receiving device 940 which receives reception optical signal 913 and converts the signal to an electric signal, an amplify circuit 941 which amplifies the electric signal output from light receiving device 940, a bandpass filter (BPF) 942 which extracts a subcarrier component of the amplified signal supplied from amplify circuit 941, a noise level detection circuit 943 which detects the noise level of the signal supplied from bandpass filter 942, an envelope detection circuit 944 which performs an envelope detection process for the signal supplied from bandpass filter 942 to eliminate the subcarrier component, a signal detection level generation circuit 945 which detects the level of the signal supplied from envelope detection circuit 944, and a comparison circuit 946 which compares the signal supplied from envelope detection circuit 946 with the signal supplied from the signal detection level generation circuit 945 to output reception electric signal 914 which has been subjected to the waveform reshaping process.
Light receiving device 940 is formed of a photodiode or the like. Noise level detection circuit 943 formed of an integrator having a long time constant detects the noise level by integrating continually generated noises for a relatively long time period. When the noise level output from noise level detection circuit 943 increases, an amplification factor is reduced to prevent saturation due to the noises under the control by amplify circuit 941.
The signal supplied from bandpass filter 942 is resistively divided according to a predetermined ratio and supplied to envelope detection circuit 944. Signal detection level generation circuit 945 having a short time constant captures the peak of the output signal from envelope detection circuit 944. The output signal from signal detection level generation circuit 945 is resistively divided according to a predetermined ratio and supplied to comparison circuit 946.
In general, a light emission intensity, namely the intensity of light radiated from the light emitting device increases as the current flowing through the light emitting device increases. However, the current flowing through light emitting device 935 in the conventional optical transmission circuit 902 is constant as shown in FIG. 4. The value of electric current of light emitting device 935 is usually determined by light emission power required for achieving a desired communication distance. For example, if the desired communication distance is 1 m, optical transmission circuit 902 always emits light with the light emission intensity which enables transmission to any optical transmission device 1 m away from itself. Therefore, if the transmission distance is shorter, the transmission is carried out with an unnecessarily stronger light emission intensity. Accordingly, unnecessarily large current flow through light emitting device 935 leads not only to wasteful power consumption, but to degradation of characteristics and shorter lifetime of the light emitting device, and consequently, the light emitting device radiates an unnecessary interference wave to another optical communication device.
Those inventions disclosed in Japanese Patent Laying-Open Nos. 6-252853 and 9-69817 solve such a problem.
An optical communication device disclosed in the Japanese Patent Laying-Open No. 6-252853 includes a light receiving device which receives an optical signal and converts the optical signal to an electric signal, an amplifier which amplifies the electric signal supplied from the light receiving device and outputs a level signal indicating the strength of the received signal, and a judge control circuit which judges the level of the received signal by the level signal supplied from the amplifier to control a driver circuit which drives a light emitting device.
An optical communication device disclosed in the Japanese Patent Laying-Open No. 9-69817 includes a first transmission and reception device, and a second transmission and reception device. The first transmission and reception device includes a light emitting device, a light emission drive control circuit which controls light emission drive of the light emitting device, and a reception unit which receives light emission intensity information transmitted from the second transmission and reception device. The second transmission and reception device includes a light receiving device, a light reception intensity detection circuit which detects the light reception intensity, namely the intensity of the light received by the light receiving device, and a transmission unit which transmits the light reception intensity information to the first transmission and reception device. The first transmission and reception device adjusts the light emission intensity of the light emitting device according to the light reception intensity information supplied from the second transmission and reception device.
Problems are described below of those inventions disclosed in the Japanese Patent Laying-Open Nos. 6-252853 and 9-69817 described above.
The first problem is that the control information of the light emission intensity is generated only from the output signal of the amplify circuit. In the communication using the light, the background light from the sunlight, inverter fluorescent lamp or the like is incident on the light receiving device to cause noise called shot noise. The sunlight called white noise leads to a noise source having an infinite extent of frequencies. The inverter fluorescent lamp leads to a noise source having frequency components reaching to several hundreds KHz.
The light receiving device photoelectrically converts the received light whether or not the incident light is a data signal or any noise. Therefore, the amplify circuit amplifies a signal in which the data signal and the noise are mixed. Even if the noise is large, in other words, even if the signal-to-noise ratio is low and a stronger light reception intensity is required, it is judged that the light reception intensity is strong by receiving the signal which is generated by amplifying the noise, and thus the light emission intensity is likely to be reduced.
The second problem is that a stable adjustment of the light emission intensity is impossible when two corresponding optical communication devices simultaneously control the emission of the light. For example, when optical communication is done at close range, the first optical communication device receives an optical signal with a high light intensity from the second optical communication device, so that the first optical communication device supposes that the second optical communication device is doing communication at a close range to transmit an optical signal to the second optical communication device with reduced light emission intensity. On the other hand, the second optical communication device receives the optical signal with the low light intensity from the first optical communication device, so that it supposes that the first optical communication device is located at a long distance to transmit an optical signal to the first optical communication device with increased light intensity. Consequently, both optical communication devices repeatedly adjust the light emission intensity, leading to erroneous adjustment of the light emission intensity by both of the optical communication devices.
The third problem is that, if the two optical communication devices have different receiving sensitivities, an efficient adjustment of the light emission intensity is impossible. For example, as bidirectional optical communication devices, the first and second optical communication devices having different receiving sensitivities are supposed to do optical communication with the same light intensity. Although there are various factors which determine the receiving sensitivity, the amplification factor is herein defined as the factor where a higher amplification factor corresponds to a superior receiving sensitivity and a lower amplification factor corresponds to an inferior receiving sensitivity. Further, it is supposed that the first optical communication device is a battery-driven equipment and adjust the light intensity, and the second optical communication device is an AC (Alternating Current)-connected equipment and does not adjust the light intensity, and that the first optical communication device has a superior receiving sensitivity and the second optical communication device has an inferior receiving sensitivity. The first optical communication device transmits an optical signal with a lower light emission intensity to the second optical communication device since the first optical communication device has the superior receiving sensitivity and is thus able to fully receive the optical signal emitted from the second optical communication device. However, the first optical communication device cannot properly receive the optical signal from the second optical communication device since the second optical communication device has the inferior receiving sensitivity, possibly leading to the state in which the signal cannot be received.
Further, it is supposed that the first optical communication device has an inferior receiving sensitivity and the second optical communication device has a superior receiving sensitivity. If the first optical communication device receives an optical signal with the minimum light reception intensity which can be received from the second optical communication device, the first optical communication device transmits an optical signal with the maximum light emission intensity to the second optical communication device. On the other hand, the second optical communication device having the superior receiving sensitivity receives an optical signal with an unnecessarily high light reception intensity, leading to waste of electric power of the first optical communication device.
The fourth problem is that no consideration is taken as to when the judgement should be made of light reception intensity, namely the intensity of the optical signal received from an optical communication device on the transmitter side. A light receiving device of the optical communication device receives, except for a data signal from any secondary station, background light such as the sunlight and the light from an inverter fluorescent lamp. Consequently, the intensity of the received light is judged on the basis of the background light incident on the optical communication device before the data signal is received from the secondary station and thus a proper control of the light emission intensity becomes impossible.
The fifth problem of the conventional art described above is that a specific circuit structure is not disclosed in those references although adjustment of the light emission intensity is described therein. For example, it is not clarified that how the transmitter section of the optical communication device adjusts the light intensity after the light emitting device receives the light emission intensity control signal. Further, the same is applied to the receiver section.